In a semiconductor device manufacturing process, a plasma etching process has been used to form a pattern on a layer formed on a semiconductor wafer serving as a target substrate. In the plasma etching process, the layer on the semiconductor wafer is etched by plasma with a resist as a mask.
Various plasma etching apparatuses have been used to perform such a plasma etching process. Currently, a capacitively coupled parallel plate type plasma etching apparatus is mainly used as a plasma etching apparatus.
In the capacitively coupled parallel plate type plasma etching apparatus, a pair of parallel plate electrodes, i.e., an upper electrode and a lower electrode, are provided within a processing chamber. A processing gas is supplied into the processing chamber, and a high frequency (RF) power is applied to at least one of the upper electrode and the lower electrode, so that a high frequency electric field is formed between these electrodes. The processing gas is excited into plasma by this high frequency electric field, and the layer on the semiconductor wafer is etched by the plasma.
As a capacitively coupled parallel plate type plasma etching apparatus, for example, a plasma etching apparatus described in Patent Document 1 has been known. This plasma etching apparatus applies a high frequency power for plasma generation having a frequency in a range of 50 MHz to 500 MHz and a high frequency power for ion attraction having a frequency in a range of 1 MHz to 4 MHz to a mounting electrode. Thus, an etching process can be carried out with high selectivity and high reproducibility. In this kind of an etching process, the etching process is carried out by using a processing gas in which positive ions predominate in the plasma.
In an etching process used in the semiconductor device manufacturing process, there has been a demand for improvement in aspect ratio to perform a process in a deeper and narrower pattern. Recently, there has been a demand for an etching process having a high aspect ratio such as HARC (High Aspect Ratio Contact) etching process which has an aspect ratio of 20 or more or a next-generation HARC etching process in which a deep trench having an aspect ratio of 40 or more can be formed.
In such a HARC etching process, if the aspect ratio is increased as the etching process progresses, positive ions are accumulated at the bottom of a hole, and, thus, an etching surface becomes positively charged. When the etching surface is positively charged, the positive ions important in accelerating the etching process cannot move straight within the hole. As a result, an etching profile may be curved or deformed. Further, as the bottom of the hole is positively charged, shading damage may be caused. Furthermore, since it becomes difficult for the positive ions to reach the bottom of the hole, an etching rate may be deteriorated. Therefore, in order to form a hole having a high aspect ratio, there has been a need for improving the conventional plasma etching apparatus.
A plasma etching apparatus satisfying the need for improvement is described in Patent Document 2. In the plasma etching apparatus described in Patent Document 2, a high frequency power supply configured to generate high frequency power for plasma generation is turned on and off on a certain cycle. Further, this plasma etching apparatus applies a negative DC voltage to an upper electrode during the on/off-period of the high frequency power supply. During the off-period of the high frequency power supply, plasma is reduced, and, thus, negative ions accelerated by the DC voltage applied to the upper electrode are introduced into the bottom of the hole, and positive charges within the hole are neutralized. After the positive charges within the hole are neutralized, if the high frequency power supply is turned on and plasma is generated, positive ions can move straight within the hole. Therefore, according to this plasma etching apparatus, a desirable etching profile can be obtained.
Further, in an etching process used in the semiconductor device manufacturing process, there has been a demand for fine pattern. In order to satisfy such a demand, an ArF photoresist, which is exposed by a laser beam with a short-wavelength ArF gas as a light emitting source, has been used as an etching mask. In the etching mask using the ArF photoresist, an opening pattern of, for example, about 0.13 μm or less may be formed.
However, since the ArF photoresist has a low plasma resistance, a surface may become rough during the etching process. Thus, vertical lines (striation) may be formed on an inner wall surface of the opening pattern, or the opening pattern may be expanded (expansion in CD). Therefore, there has been a need for improving the etching selectivity.
A plasma etching apparatus satisfying the need for improvement is described in Patent Document 3. In the plasma etching apparatus described in Patent Document 3, a negative DC voltage is applied to an upper electrode. If the negative DC voltage is applied to the upper electrode, when plasma is generated, electrons are generated around the upper electrode. These electrons are accelerated in a direction toward a target substrate on which an ArF photoresist is formed by a potential difference between a potential of the upper electrode and a potential of the plasma. If the electrons are irradiated to the ArF photoresist, a polymer structure of the ArF photoresist is changed and an etching resistance is increased, so that the etching selectivity is increased.
However, a modification (reforming) effect on an organic mask caused by irradiating the electrons depends on a thickness of a plasma sheath on the target substrate. That is, if a thick plasma sheath is formed on the target substrate, the electrons are reflected from the plasma sheath and an electron irradiation amount is decreased. Therefore, in order to modify an organic mask, there has been a need for increasing electrons to be irradiated to the organic mask.
A plasma etching apparatus satisfying the need for the increasing is described in Patent Document 4. In the plasma etching apparatus described in Patent Document 4, a high frequency power supply is turned on and off on a certain cycle. Further, this plasma etching apparatus includes a DC power supply unit including a first DC power supply unit configured to generate a first negative DC voltage and a second DC power supply unit configured to generate a second negative DC voltage having a higher absolute value than that of the first DC voltage. Further, during the on-period of the high frequency power supply, the first DC power supply unit is connected to an upper electrode. Furthermore, in this plasma etching apparatus, during the off-period of the high frequency power supply, plasma is reduced and a plasma sheath becomes thin. During this period, a negative DC voltage having a relatively high absolute value is applied to the upper electrode, so that more electrons are irradiated to an organic mask. Therefore, according to the plasma etching apparatus described in Patent Document 4, a modification effect on the organic mask can be further improved.